With advances in the widespread acceptance of CATV (cable television) systems and the like of recent years accompanied by a large number of channels for offering varieties of information, the trend is giving rise to a need for a single audio-video data recording and reproducing apparatus which, unlike conventional VTRs (video tape recorders), records or reproduces simultaneously a plurality of channels of audio-video data or reproduces one channel of data while recording another channel of data. The need is being met by growing popularization of an apparatus called a video server (or AV (Audio and/or Video) server) for recording and reproducing audio and video data using nonlinear accessible storage medium such as hard disks.
Generally, video servers used by broadcasting stations need to have high data transfer rates to meet rigorous requirements of image and sound qualities, as well as large storage capacities for recording data for extended periods of time. Attempts to meet the above needs include the use of data recording and reproducing equipment comprising a plurality of hard disk (hereinafter called "HD") drives which may store video and audio data and which are accessible in parallel. Further, attempts to meet the needs for ensuring reliability include recording parity data in the event of failure of any HD drives.
A multi-channel video server is implemented which is capable of addressing diverse modes of use such that a VOD (video on demand) system or an NVOD (near video on demand) system is constituted, by recording raw data composed of a plurality of items of audio and video data in a distributed fashion so as to output multi-channel data simultaneously or reproducing the same raw data in a temporally staggered manner for multi-channel reproduction, even if a number of data channels required by the contents of programs or broadcasts offered by broadcasting stations is different.
The above type of video server employs a data recording and reproducing apparatus based on RAID (redundant arrays of inexpensive disks) techniques involving the use of a plurality of hard disk drives (called HDDs hereunder) each comprising a plurality of HDs, proposed in 1988 by Patterson et al., in "A Case for Redundant Arrays of Inexpensive Disks (RAID)", ACM SIGMOND Conference, Chicago, Ill., Jun. 1-3, 1998.
In the paper cited above, the RAID techniques are grouped into five categories: RAID-1 through RAID-5. RAID-1 is a technique for writing the same contents to two HDDs. RAID-3 is a technique for dividing input data into equal lengths and writing the divided data to a plurality of HDDs while generating parity data and writing the generated parity data to another HDD, the parity data being acquired by exclusive-OR'ing corresponding data blocks between HDDs. RAID-5 is a technique for enlarging data divisions (blocks) and writing each data division as a data block to one HDD, whereby the result of exclusive-OR'ing any two corresponding data blocks between HDDs (i.e., parity data) is written as a parity block to another HDD as well as to further HDDs in a distributed manner.
When reading data from at least two discontinuous areas of HDDs illustratively over a predetermined period of time, the above type of AV server (video server) reads the data in an order designated by a command.
Where data are read from the HDD areas in the command-designated order, the area order may or may not be advantageous for access to the HDD areas.
That is, when area order designated by command coincides with address order of the areas in the HDDs on which the data are recorded, reading the data from the areas in the order designated by command may be advantageous of less amount of access time. This is because the time to seek the HDDs in relation to access to the areas is minimized as head is moved according to the address order when reading the data.
On the other hand, when area order designated by command does not coincide with address order of the areas in the HDDs on which the data are recorded, the order designated by the command is disadvantageous because the head may not access the areas in the order designated by the command and then, the traveling distance of head over the areas is extended and the HDD seek time is prolonged.
It is therefore an object of the present invention to provide an apparatus for reading data, data input/output device and others whereby the efficiency of data transfer is enhanced.